Information as to the distance between a vehicle and a fixed location such as an aircraft in the vicinity of an airfield, and the airfield itself, is available from equipment which is well known to those skilled in the art. Because of the need for many different aircraft to cooperate with common airfield located equipment to provide this information, the Federal Government regulates the characteristics of such equipment. In addition to the need to minimize bandwidth requirements of such equipment because of competing needs of other RF spectrum users, the DME field is further constrained by the need for equipment to be able to correctly respond to multiple transponders, with messages being transmitted at random times with respect to other messages. To satisfy these stringent requirements, a frequency band in the range 962-1213 MHz. has been set aside for DME operation. DME operation proceeds on an interrogation-reply protocol. An interrogating aircraft transmits, at an assigned frequency, a pulse pair. Upon reception of this interrogation message at the airfield, a reply pulse pair is transmitted. When a reply is received at the interrogating aircraft, delay between the interrogation signal and the reply is used to determine range. The reply signal is offset in frequency from the interrogation signal by a fixed frequency offset. This use of a frequency offset allows the interrogating aircraft to select only the reply generated in response to its own interrogation. Since range is determined by the time difference between interrogation and the corresponding reply it should be apparent that reception of spurious replies may well result in developing erroneous range information and therefore, stringent requirements are placed upon the frequency spectrum of both the interrogation message and the reply message.
As those skilled in the art will be aware, such stringent spectrum requirements result in signals have shapes which are not ideal from the standpoint of generating precise timing points from which measurements can be effected to determine range.
In view of the foregoing, it is quite important for the receiver to be able to detect, within a limited tolerance, the time of arrival of the reply message. Since the reply message is received at RF, the receiver requires apparatus to detect the RF signal and to generate from it a signal which can be employed for the processing necessary to derive range information. So long as the detected signal and the RF signal have a linear relationship, signal processing effected at the receiver will produce an output which can be used as indicating distance to the airfield. Any non-linearity in the relationship between the RF input and the receiver generated replica will obviously lead to errors in the distance indication, and therefore, such non-linearities are sought to be minimized.
It is therefore one object of the present invention to provide, in a DME receiver, an apparatus to produce, from the received RF input, a linear replica, at baseband of the RF signal. It is another object of the present invention to maintain the linearity between the baseband signal and the RF input, over a wide dynamic range.